Power supply controller responsive to a feedforward signal

ABSTRACT

An apparatus and method of switching a switch of a power supply in response to an input voltage signal are disclosed. According to aspects of the present invention, a power supply controller includes a switch duty cycle controller coupled to receive a feedback signal and a duty cycle adjust signal. The switch duty cycle controller is coupled to generate a drive signal coupled to control switching of a switch, which is coupled to an energy transfer element, to regulate energy delivered from an input of a power supply to an output of the power supply. The power supply controller also includes a gain selector circuit coupled to receive an input voltage signal, which is representative of an input voltage to the power supply, to generate the duty cycle adjust signal received by the switch duty cycle controller. The maximum duty cycle of the drive signal to be varied in response to a plurality of linear functions over a range of values of the input voltage signal.

BACKGROUND INFORMATION

1. Field of the Invention

The present invention relates generally to control circuits and, morespecifically, the present invention relates to control circuits used inpower supplies that are responsive to a feedforward signal.

2. Background

Power supply control circuits may be used for a multitude of purposesand applications. Most power converter applications have both cost andperformance goals. It is therefore necessary for control circuitfeatures to be implemented that minimize the cost of external circuitrysuch as the input bulk capacitor in AC/DC converter power supplies. Inaddition the tolerances of the control circuit are important to provideconsistent performance in the power supply application as well asfurther reduce power converter cost by reducing the design margins thatneed to be applied when the power converter is developed.

Power supplies typically comprise a power supply controller circuit, aswitch coupled to an energy transfer element, a source of input voltageand one or more outputs. The power supply controller typically controlsa switching of the switch to regulate energy delivered from the input tothe output of the power converter in response to a feedback signalgenerated by a feedback circuit forming part of the power converter.Power supply controller circuits operating with pulse width modulator(PWM) modes of operation regulate the duty cycle of the switch as onetechnique to regulate energy delivered from the input to the output ofthe power supply. The duty cycle of the switch is the ratio of theswitch on time to an overall switching cycle period defined by the powersupply controller circuit.

Power supply controllers make use of feedforward signals in common powerconverter topologies such as forward converters and flyback converters.A feedforward signal is a signal whose magnitude is a function of thevalue of the input voltage to the power converter. In general thereforea feedforward signal can be regarded an input voltage signal that isrepresentative of an input voltage to a power supply. A feedforwardsignal is typically used to provide a way to adjust the switch dutycycle independent of the feedback signal or in other words for a givenor fixed feedback signal. In flyback converters, the advantage of afeedforward signal can for example be to reduce the size of a bulkcapacitance at the input of an AC/DC power converter in particular forpower supply controllers operating in the voltage mode of control whereripple voltage across the bulk capacitor is more difficult to filter.The ability to adjust the switch duty cycle independent of the feedbacksignal allows fast response to ripple voltage appearing across the bulkcapacitor and therefore reduces ripple voltage appearing across theoutput of the power converter. Without the use of feedforwardtechniques, the power supply controller must respond to the feedbacksignal that responds to ripple voltage appearing across the powerconverter output and the power supply controller then controls the dutycycle of the switch accordingly to reduce the power converter outputripple voltage. This introduces delays and output ripple is thereforedifficult to reduce without for example increasing the size of the inputbulk capacitor. In forward converters a feedforward signal is typicallyused by the power supply controller to control the maximum duty cycle ofthe switch to ensure there is sufficient time to reset the flux in themagnetic core of the energy transfer element as will be known to oneskilled in the art.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting and non-exhaustive embodiments of the present invention aredescribed with reference to the following figures, wherein likereference numerals refer to like parts throughout the various viewsunless otherwise specified.

FIG. 1 is a schematic illustrating generally an example power converteremploying a power supply controller circuit responsive to a feedforwardsignal and a feedback signal in accordance with the teachings of thepresent invention.

FIG. 2 shows generally duty cycle control waveforms for an example powersupply controller circuit responsive to a feedforward signal inaccordance with the teachings of the present invention.

FIG. 3 shows generally maximum duty cycle control waveforms for anexample power supply controller circuit responsive to a feedforwardsignal in accordance with the teachings of the present invention.

FIG. 4 shows generally duty cycle for a fixed value of feedback signalcontrol waveforms for an example power supply controller circuitresponsive to a feedforward signal in accordance with the teachings ofthe present invention.

FIG. 5 shows generally an example power supply controller circuitresponsive to a feedforward signal and a feedback signal in accordancewith the teachings of the present invention.

FIG. 6 is a schematic illustrating generally an example of a portion ofa power supply controller circuit responsive to a feedforward signal anda feedback signal in accordance with the teachings of the presentinvention.

FIG. 7 is a schematic illustrating generally an example of a portion ofa power supply controller circuit responsive to a duty cycle adjustsignal and a feedback signal in accordance with the teachings of thepresent invention.

FIG. 8 shows generally maximum duty cycle control waveforms for anexample power supply controller circuit responsive to a feedforwardsignal in accordance with the teachings of the present invention.

FIG. 9 shows generally duty cycle for a fixed value of feedback signalcontrol waveforms for an example power supply controller circuitresponsive to a feedforward signal in accordance with the teachings ofthe present invention.

FIG. 10 is a schematic illustrating generally an example of a portion ofa power supply controller circuit responsive to a feedforward signal anda feedback signal in accordance with the teachings of the presentinvention

DETAILED DESCRIPTION

Methods and apparatuses for implementing a power supply controllercircuit responsive to a feedforward signal are disclosed. In thefollowing description numerous specific details are set forth in orderto provide a thorough understanding of the present invention. It will beapparent, however, to one having ordinary skill in the art that thespecific detail need not be employed to practice the present invention.In other instances, well-known materials or methods have not beendescribed in detail in order to avoid obscuring the present invention.

Reference throughout this specification to “one embodiment,” “anembodiment,” “one example,” “an example” or the like means that aparticular feature, structure or characteristic described in connectionwith the embodiment or example is included in at least one embodiment orexample of the present invention. Thus, appearances of the phrases “inone embodiment,” “in an embodiment,” “one example” or “an example” invarious places throughout this specification are not necessarily allreferring to the same embodiment or example. Furthermore, the particularfeatures, structures or characteristics may be combined in any suitablecombinations and/or subcombinations in one or more embodiments orexamples. In addition, it is appreciated that the figures providedherewith are for explanation purposes to persons ordinarily skilled inthe art and that the drawings are not necessarily drawn to scale.

As mentioned above, power supply controller circuits operating with PWMmodes of operation regulate the duty cycle of a drive signal driving theswitching of the switch as one technique to regulate energy deliveredfrom the input to the output of the power supply, where the duty cycleof the switch is the ratio of the switch on time to an overall switchingcycle period defined by the power supply controller circuit. The optimummaximum duty cycle or, duty cycle of the drive signal for a givenfeedback signal, is a non-linear function of the power converter inputvoltage. Known circuitry required to provide this non-linear functiontypically relies either on an external capacitor to the power supplycontroller, which adds cost and typically has poor tolerance or anintegrated capacitor, which also has poor tolerance from unit to unitand adds silicon area therefore adding cost to the power converterdesign. Circuitry to apply a linear function of maximum duty cycle or,duty cycle for a given feedback signal, as a function of power converterinput voltage provides optimum performance at only one input voltagelevel and therefore compromises the power converter design at all othervalues of input voltage.

A power supply controller circuit responsive to a feedforward signal inaccordance with the teachings of the present invention will now bedescribed. Examples of the present invention involve methods andapparatuses to generate a power supply controller circuit responsive toa feedforward signal. Example power supply control circuits inaccordance with the teachings of the present invention provide anapproximation to the ideal feedforward function of duty cycle versuspower converter input voltage that retains the system level savingsprovided by controlling the switch duty cycle in response to afeedforward signal, while maintaining a power supply control circuitdesign, which is low cost and provides adequate control of unit to unittolerance such that system level savings are retained in accordance withthe teachings of the present invention.

FIG. 1A shows generally a schematic of one example of a power converter100 employing a power supply controller responsive to a feedforwardsignal in accordance with the teachings of the present invention. In theexample, power converter 100 is a flyback converter. It is noted that inother examples, power converter 100 could also be one of many powerconverter configurations such as a forward converter and could be anisolated or non-isolated converter in accordance with the teachings ofthe present invention.

The example power supply in FIG. 1 provides output power to a load 165from a DC input voltage V_(IN) 105. In the example, a bulk capacitor 195is coupled across the input to the power converter 100. In applicationswhere the input voltage 105 is generated from an AC source that is notshown, this bulk capacitor 195 acts as a low pass filter storing energyto maintain the input voltage 105 at a level that allows the powerconverter 100 to supply the required output power to load 165 betweenthe AC line cycles. As such, in one example, the voltage filteredrectified voltage across bulk capacitor 195 has a ripple voltage ofmagnitude typically less than 25% of the peak value of the DC voltageacross capacitor 195 though the actual value of which is dependent onthe value of the bulk capacitor 195 along with other operatingconditions of the power converter 100.

The input voltage V_(IN) 105 is coupled to an energy transfer element T1175 and a switch S1 125. In the example of FIG. 1A, the energy transferelement T1 175 is coupled between an input of the power supply and anoutput of the power supply. In the example of FIG. 1, the energytransfer element T1 175 is illustrated as a transformer with one outputwinding 176. In other examples, the transformer may have more than oneoutput winding, with additional windings to provide power to additionalloads, to provide bias voltages, or to sense the voltage at a load.

As shown in the illustrated example, a clamp circuit 110 is coupled tothe primary winding of the energy transfer element T1 175 to limit themaximum voltage across the switch S1 125. Power supply controller 145 iscoupled to generate a drive signal coupled to be received by the switch125 to control a switching of the switch 125 to regulate energydelivered from the input to the output of the power supply in responseto a feedback signal 155 generated by a feedback circuit 160.

In one example, switch S1 125 is a transistor such as for example apower metal oxide semiconductor field effect transistor (MOSFET). In oneexample, controller 145 includes integrated circuits and discreteelectrical components. In one example switch S1 125 and power supplycontroller 145 form part of a monolithic integrated circuit. In another,example switch S1 125 and power supply controller 145 form part of ahybrid integrated circuit where for example they are separate silicondie but housed in the same integrated circuit package. The switching ofswitch S1 125 produces pulsating current I_(D) 120 flowing through theinput winding 177 of energy transfer element T1 175 as illustrated inFIG. 1B. Energy stored in the energy transfer element during the on timeof switch 125, is transferred to output capacitor 135 during the switch125 off time.

As shown in the depicted example, the power converter output quantity tobe regulated is U_(O) 150, which in general could be an output voltageV_(O), an output current I_(O), or a combination of the two. Theregulated quantity is not necessarily fixed, but can be regulated tochange in a desired way through the design of the feedback circuit 160.For example, in one example, the output U_(O) changes from an outputvoltage to an output current in response to the magnitude of the outputvoltage or the output current. Feedback circuit 160 is coupled toreceive the output quantity U_(O) 150 to produce a feedback signalU_(FB) 155, which is coupled as an input signal to the power supplycontroller 145.

As illustrated in the example of FIG. 1A, another input signal to thecontroller 145 is the feedforward signal U_(FF) 170, which is an outputof a feedforward circuit 140. The feedforward circuit 140 is coupled toreceive a signal U_(I) 115, which is a function of power converter 100input voltage V_(IN) 105. In the example, both U_(I) 115 and U_(FF) 170may be either voltage or current signals. In one example, signals U_(I)115 and U_(FF) 170 are the same signal, where, for example, feedforwardcircuit 140 includes a resistor.

In the illustrated example, it is appreciated that FIG. 1A shows twopossible ways to generate signal U_(I) 115. In one example, signal U_(I)115 may be generated by coupling feedforward circuit 140 directly to thepower converter 100 input voltage V_(IN) 105. In another example, signalU_(I) 115 may be generated by coupling feedforward circuit 140 to theoutput of a circuit 185, which includes a forward winding 178 of energytransfer element 175. The signal generated by forward winding 178 isrectified and smoothed by components 179 and 181. In the example, thevoltage appearing across winding 178 during the switch 125 on time isdirectly proportional to the input voltage V_(IN) 105 and can thereforebe used to generate signal 115 to be applied to feedforward circuit 140in accordance with the teachings of the present invention.

FIG. 1B also illustrates generally an example waveform for current I_(D)120 to show the parameters that the controller can adjust to regulatethe output quantity U_(O) 150. As shown in the example, the maximum ofcurrent I_(D) 120 is I_(MAX) 121, the switching period is T_(S) 122, andthe duty cycle is D 124. The controller typically limits the duty cycleto a maximum D_(MAX) that is less than 100%. In one example, controller145 includes an oscillator that defines a substantially regularswitching period T_(S) 122. In one example, regulation is accomplishedby control of the switch S1 125 on time within a switching period. Ineach switching period, the fraction of the switching period that theswitch is on is the duty cycle (D) of the switch. In one example,regulation is accomplished by control of the maximum current I_(MAX) 121of the switch. In another example, regulation is accomplished by controlof the switching period T_(S) 122. Regardless of whether I_(MAX) 121 orT_(S) 122 is controlled, the switch on time as a fraction of theswitching period is modulated and therefore the regulation mode can beregarded as a duty cycle mode of control in accordance with theteachings of the present invention.

FIG. 2 shows generally one example of duty cycle control waveforms thatin one example could be applied by controller 145 to control theswitching of switch S1 125 in response to feedback signal 155 andfeedforward signal 170. In general, the four characteristics 217, 216,215 and 214 correspond to the value of V_(IN) 105 increasing betweenvalues V₁, V₂, V₃ and V₄, respectively, as indicated by arrow 218. Forany fixed input voltage value, the maximum duty cycle and duty cycle fora given value of the feedback signal U_(FB) 219 are defined by aspecific duty cycle 220 versus feedback signal 219 characteristic.However, as the feedforward signal magnitude increases, both maximumduty cycle and duty cycle for a given value of the feedback signal 219reduce.

In one example, the magnitude of the feedforward signal increasingcorresponds to the power converter input voltage increasing. In theexample, for a fixed feedback signal value U_(X) 211, the power supplycontroller 145 varies the switch 125 duty cycle between values D_(X1)206 to D_(X2) 207, D_(X3) 208 and D_(X4) 209 as the input voltage variesfrom V₁ to V₂, V₃ and V₄ respectively. The variation of duty cycle ofswitch 125 between D_(X1) 206 to D_(X4) 209 as the input voltage variesfrom V₁ to V₂, V₃ and V₄, is therefore independent of feedback signalU_(FB) 155. In the example, the slope of duty cycle versus feedbackcurrent, when the feedback signal exceeds the value U_(B) 210, isunresponsive to the value of the feedforward signal and remains a fixedslope −m_(FB) 213 until duty cycle value D_(MIN) 205 is reached. Inother examples, the slope 213 could be non-linear in nature inaccordance with the teachings of the present invention.

FIG. 3 shows generally one example of the relationship between maximumduty cycle D_(MAX) 302 and feedforward or input voltage signal U_(FF)301 magnitude that in one example could be applied by controller 145 tocontrol the switching of switch S1 125 in response to feedforward signal170. In the example, the maximum duty cycle of the drive signalcontrolling the switch is varied in response to a plurality of linearfunctions over a range of values of the input voltage signal orfeedforward signal in accordance with the teachings of the presentinvention. For instance, in the illustrated example, the varying of themaximum duty cycle is a first linear function of the input voltagesignal when the input voltage signal is below a first threshold valueV_(BR1) 303. The varying maximum duty cycle is a second linear functionof the input voltage signal when the input voltage signal is between thefirst threshold value V_(BR1) 303 and a second threshold value V_(BR2)304. The varying of the maximum duty cycle is a third linear function ofthe input voltage signal when the input voltage signal is between thesecond threshold value V_(BR2) 304 and a third threshold value V_(BR3)305.

As shown in the example, the slopes of the first, second and thirdlinear functions are slope 1 306, slope 2 307 and slope 3 308,respectively. In one example, slope 1 306 has a slope substantiallyequal to zero, though in other examples slope 1 306 could also have anon-zero slope while in accordance with the teachings of the presentinvention. One reason slope 1 306 could have a slope substantially equalto zero is that the power supply controller 145 may have to limit theabsolute maximum duty cycle that can ever be applied to the switching ofswitch 125 for practical reasons related to the design of the powersupply controller 145 but also to limit stress on the switch 125 underpower supply fault conditions for example.

In one example maximum duty cycle values D_(MAX1) 309, D_(MAX2) 310,D_(MAX3) 311 and D_(MAX4) 312 correspond to D_(MAX1) 201, D_(MAX2) 202,D_(MAX3) 203 and D_(MAX4) 204, respectively, in FIG. 2. In the example,first, second and third linear functions of the input voltage signal arelinear reductions in maximum duty cycle as the input voltage signalincreases. In another example, slope 3 308 could also have a slopesubstantially equal to zero in accordance with the teachings of thepresent invention.

FIG. 4 shows generally one example of the relationship between dutycycle 402 for a given or fixed value of feedback signal U_(FB) andfeedforward U_(FF) or input voltage signal 401 magnitude, which in oneexample could be applied by controller 145 to control the switching ofswitch S1 125 in response to feedforward signal 170. In one example, thegiven or fixed value of the feedback signal could be U_(X) 211 in FIG. 2though it is appreciated that the given of fixed value of feedbacksignal could be any value of the feedback signal greater than U_(B) 210in FIG. 2.

In the example, the duty cycle of the drive signal controlling theswitch for a fixed value of the feedback signal is varied in response toa plurality of linear functions over a range of values of the inputvoltage signal or feedforward signal in accordance with the teachings ofthe present invention. For instance, in the illustrated example, thevarying of the duty cycle for the fixed value of the feedback signal isa first linear function of the input voltage signal when the inputvoltage signal is below a first threshold value V_(BR1) 403. The dutycycle of the switch is a second linear function of the input voltagesignal when the input voltage signal is between the first thresholdvalue V_(BR1) 403 and a second threshold value V_(BR2) 404. The dutycycle of the switch is a third linear function of the input voltagesignal when the input voltage signal is between the second thresholdvalue V_(BR2) 404 and a third threshold value V_(BR3) 405.

As illustrated in the example, the slopes of the first, second and thirdlinear functions are slope 1 406, slope 2 407 and slope 3 408,respectively. In one example, slope 1 406 has a slope substantiallyequal to zero though other examples, it is appreciated that slope 1 406could also have a non-zero slope in accordance with the teachings of thepresent invention. In one example duty cycle values D_(X1) 409, D_(X2)410, D_(X3) 411 and D_(X4) 412 correspond to D_(X1) 206, D_(X2) 207,D_(X3) 208 and D_(X4) 209, respectively, in FIG. 2. In the example,first, second and third linear functions of the input voltage signal arelinear reductions in duty cycle as the input voltage signal increases.In another example, slope 3 408 could also have a slope substantiallyequal to zero in accordance with the teachings of the present invention.

FIG. 5 shows one example of an internal block diagram of a power supplycontroller 545 including a switch duty cycle controller 522 coupled to afeed forward gain selector circuit 530 in accordance with the teachingsof the present invention. In the example, feed forward gain selectorcircuit 530 is coupled to receive feedforward signal I_(FF) 510. In oneexample, the input DC voltage V_(IN) 505 is equivalent to V_(IN) 105 inFIG. 1. In one example, switch S1 525 is equivalent to switch 125 inFIG. 1 and the current flowing through switch 525 I_(D) 520 isequivalent to I_(D) 120 in FIG. 1.

As shown in the example, V_(IN) 505 is coupled to feedforward circuit540, which in the example is illustrated as a resistor R_(FF). Inanother example, feedforward circuit 540 could be a resistor divider andfeedforward signal 510 could be a voltage signal. In the example, thecurrent I_(FF) 510 flowing through feedforward circuit 540 is afeedforward or input voltage signal, which in one example is equivalentto signal U_(FF) 170 in FIG. 1. Therefore, as V_(IN) 505 increases,feedforward signal I_(FF) 510 also increases.

In the example, feedforward I_(FF) or input voltage signal 510 iscoupled to gain selector circuit 530. Gain selector circuit 530generates a duty cycle adjust signal 535 that is coupled to switch dutycycle controller circuit 522. In the example, switch duty cyclecontroller circuit 522 is also coupled to receive feedback signal U_(FB)555. Gain selector circuit 530 selects the gain applied to thefeedforward signal 510. The gain applied determines the characteristicof maximum duty cycle as a function of input voltage V_(IN) 505 asillustrated in FIG. 3. The gain applied also determines thecharacteristic of duty cycle for a given or fixed value of feedbacksignal as a function of input voltage V_(IN) 505 as illustrated in FIG.4. Since in FIG. 3 and FIG. 4 the slope of the characteristics 313 and413 change depending on the magnitude of the input voltage, gainselector circuit 530 selects the gain multiplier m_(FF) of term 536according to the value of the feedforward signal I_(FF) 510 inaccordance with the teachings of the present invention. The circuitry toperform this function is described in FIG. 6 below.

FIG. 6 shows generally an example schematic of a gain selector circuit630 coupled to receive a feedforward I_(FF) or input voltage signal 610at terminal 651. Gain selector circuit 630 generates a signal 635 thatis coupled to be received by a switch duty cycle controller circuitdiscussed below with reference to FIG. 7. Transistor 652 and voltagesource 653 set the voltage at terminal 651, which for example allows thevalue of resistor 540 in FIG. 5 to be accurately calculated to provide adesired value of feedforward signal 510 for a given input voltage 505.Current source 632 limits the maximum current that can flow throughtransistor 652 to ensure the stability of the voltage at terminal 651under normal operating conditions.

As shown in the example, the feedforward current 610 is mirrored bycurrent mirror 601. In the example, the current mirror 601 sets a ratiom1:1, which reduces the mirrored current to a lower value than thefeedforward current 610 to reduce internal power consumption of gainselector circuit 630. The current is again mirrored by one to onecurrent mirror 615 to generate two equal currents 612 and 611 of valueI_(FF)/m1. The RC filter including of capacitor 605 and resistor 610provides noise filtering. In one example, current source I_(B2) 645 isgreater in value than current source 640 I_(B1).

In the example, for values of I_(FF)/m1 611 less than or equal to I_(B1)640, I₁ 625 and therefore m_(FF)I_(FF) 635, which substantially equal toI₁ 625 through the action of one to one current mirror 691, aresubstantially zero. For values of I_(FF)/m1 611 and 612 greater thanI_(B1) 640 but less than or equal to I_(B2) 645, I₁ 625 and thereforeduty cycle adjust signal m_(FF)I_(FF) 635 is substantially equal to(I_(FF)/m1−I_(B1)). For values of I_(FF)/m1 611 and 612 greater thanI_(B2) 645, m_(FF)I_(FF) 635 is substantially equal to(I_(FF)/m1−I_(B1))−(I_(FF)/m1−I_(B2))m2. Where m2 is the ratio ofcurrent mirror 620, which is applied to current I₂ 631 before it issubtracted from current I₁ 625. In general therefore the followingrelationship is true:m _(FF) I _(FF) =I ₁ −I ₂ m2  (1)The relationships above are also summarized in box 650 of FIG. 6.

In the example, the gain applied to feedforward signal I_(FF) 610 bygain selector circuit 630 has three sections. The variation therefore ingain of duty cycle adjust signal 635 depending on the relative values ofI₁ and I₂ which is coupled to switch duty cycle controller in oneexample provides the characteristics illustrated in FIG. 2 and FIG. 3.In one example, the condition where I_(FF)/m1 611 is less than or equalto I_(B1) 640 corresponds to slope 1 306 and 406 in FIGS. 3 and 4respectively. In one example, the condition where I_(FF)/m1 611 isgreater than I_(B1) 640 but less than or equal to I_(B2) 645 correspondsto slope 2 307 and 407 in FIGS. 3 and 4 respectively. In one example,the condition where I_(FF)/m1 611 is greater than I_(B2) 645 correspondsto slope 3 308 and 408 in FIGS. 3 and 4 respectively. The way in whichsignal m_(FF)I_(FF) 635 is processed by the switch duty cycle controllercircuit is described below with reference to FIG. 7.

FIG. 7 shows generally a schematic of a portion of a power supplycontroller circuit 700. The circuit 700 is coupled to receive a feedbacksignal I_(FB) 701, which in one example is equivalent to feedback signal555 in FIG. 5. The circuit 700 is coupled to receive a duty cycle adjustsignal 735 which in one example is equivalent to duty cycle adjustsignal 535 in FIG. 5. The output of circuit 700 is the V_(PWM) signal765, which in one example is used to derive a drive signal 521 to drivethe switching of switch 525 in FIG. 5.

When oscillator 702 output signal 720 is low, switch 799 is closed andcapacitor 780 is charged at a rate determined by the sum of currentsource I₁ 745 and I 1738. This corresponds to the rising edge ofwaveform 796, which is a representation of voltage V_(D) 795 over time.During the time where oscillator 702 output signal 720 is low, outputsignal 774 from AND gate 761 is low as is V_(PWM) signal 765corresponding in one example to the time when switch 525 in FIG. 5 isoff.

When oscillator 702 output signal 720 goes high, switch 799 is openedand charging of capacitor 780 is stopped. After a delay perioddetermined by rising edge delay circuit 764, switch 785 is closed,causing the capacitor C_(D) 780 to be discharged at a rate determined bycurrent source 782. This corresponds to the falling region of waveform796. The flat top portion of waveform 796 is caused by the delay betweenswitching switch 799 off and switching switch 785 on.

The time taken for the voltage of waveform 796 to fall below referencevoltage level 772, determines the on time Ton of V_(PWM) output 765.Since the discharge rate of capacitor 780 is fixed by current source782, the on time Ton of V_(PWM) output 765 is determined by the rate atwhich capacitor 780 was charged during the period that switch 799 wasclosed. This in turn is a function of I 738 and I₁ 745. The on time Tonof V_(PWM) output 765 as a proportion of the overall cycle time Ts isthe duty cycle and in one example corresponds to the duty cycle ofswitch S1 525 in FIG. 5.

The PWM gain of the controller duty cycle characteristic as a functionof feedback signal 701 is set by the relative magnitudes of currentsources 736, 745 and 782 and in one example corresponds to the slope 213in FIG. 2. However since this is not relevant to the teachings of thepresent invention, this is not discussed further here.

Examples in accordance with the teachings of the present inventionrelate to the influence of duty cycle adjust signal 735 on the dutycycle for a given feedback signal 701 value and the maximum duty cycleof for example switch 525 in FIG. 5. It is understood that other factorssuch as the current 520 flowing through switch 525 could also influencethe actual on time of switch 525 in certain conditions. For example, ifthe power supply is operating in a fault condition where the current 520is sensed to have exceeded safe levels, the on time of switch 525 couldbe terminated for this reason rather than the V_(PWM) 765 signal goinglow in FIG. 7.

As shown in FIG. 7, relationship 739 governs the influence of feedbacksignal 701 and duty cycle adjust signal 735 on the charging of capacitor780 and therefore the duty cycle of V_(PWM) output signal 765. When dutycycle adjust signal 735 is substantially zero, in one examplecorresponding to a value of I₁ 625 of substantially zero in FIG. 6,relationship 739 becomes:I=PI ₀ −m _(c)(I _(FB) −I _(B))  (2)In one example the relationship of equation 2 gives rise to acharacteristic of duty cycle versus feedback signal 701 as defined bycharacteristic 217 in FIG. 2.

For all conditions when I_(FF)/m1 611 in FIG. 6 is greater than I_(B1)640 duty cycle adjust signal 735 is finite. Relationship 739 is then:I=PI ₀ −{m _(c)(I _(FB) −I _(B))+m _(FF) I _(FF)}  (3)The value of the slope of the duty cycle adjust signal 735 as a functionof feedforward signal 610 is fixed and linear for each of the range ofinput voltage conditions as described with reference to FIGS. 5 and 6above. For each condition when duty cycle adjust signal is non-zero, therelationship of equation 3 and 739 applies to determine the charge rateof capacitor 780.

It should be noted that the relationship in equation 3 and 739 is onlytrue for values of feedback signal I_(FB) 701 equal to or greater thancurrent source I_(B) 703. For values of feedback signal I_(FB) 701 isless than current source I_(B) 703, term 741 in relationship 739 issubstantially zero, but does not go negative. This is due to the actionof current mirror 752 whose output current cannot be less than zero.

In one example, the condition where feedback signal I_(FB) 701 is lessthan current source I_(B) 703 corresponds to region 251 FIG. 2, wherechanges in feedback signal 219 have no influence on the duty cycle D 220which stays at a maximum value. However, increasing feedforward signalmagnitude reduces the maximum duty cycle as illustrated by D_(MAX)values 201, 202, 203 and 204. In one example, the value of feedbacksignal U_(B) 210 that is required to influence the duty cycle D is fixedindependent of the magnitude of the feedforward signal. In one examplethis corresponds to the circuit of FIG. 7 where the term 741 ofrelationship 739 is zero until the value of the feedback current I_(FF)701, is greater than current source I_(B) 703.

FIG. 8 shows generally one example of the relationship between maximumduty cycle 802 and feedforward or input voltage signal 801 magnitudethat in one example could be applied in FIG. 5 by power supplycontroller 545 to control the switching of switch S1 525 in response tofeedforward signal 510 in accordance with the teachings of the presentinvention.

In the example, the maximum duty cycle 802 is a plurality of linearfunctions of the input voltage signal 801 in accordance with theteachings of the present invention. In the example, the maximum dutycycle of the switch is a first linear function of the input voltagesignal 801 when the input voltage signal is within a first range ofvalues 820. The maximum duty cycle 802 of the switch is a second linearfunction of the input voltage signal 801 when the input voltage signal801 is within a second range of values 823. The maximum duty cycle 802of the switch is an nth linear function of the input voltage signal 801when the input voltage signal is within an nth range of values 826.

In one example, slope 1 827 has a slope substantially equal to zerothough it is understood that in other examples slope 1 827 could alsohave a non-zero slope in accordance with the teachings of the presentinvention. In one example, all linear functions other than that whilethe input voltage signal 801 is within a first range of values 820 arelinear reduction of the maximum duty cycle 802 as the input voltagesignal 801 increases. In another example, any one or more of the nslopes could have a slope substantially equal to zero as long as one ofthe n slopes has a linear reduction of the maximum duty cycle 802 as theinput voltage signal 801 increases whilst still benefiting from theteachings of the present invention.

FIG. 9 shows generally one example of the relationship between the dutycycle for a given or fixed value of feedback signal U_(FB) 902 andfeedforward or input voltage signal 901 magnitude that in one examplecould be applied in FIG. 5 by power supply controller 545 to control theswitching of switch S1 525 in response to feedforward signal 510 for afixed feedback signal value U_(FB) 555. In the example the duty cyclefor a fixed value of feedback signal U_(FB) 902 is a plurality of linearfunctions of the input voltage signal 901.

In the example, the duty cycle for a fixed value of feedback signalU_(FB) 902 of the switch is a first linear function of the input voltagesignal 901 when the input voltage signal is within a first range ofvalues 920. The duty cycle for a fixed value of feedback signal U_(FB)902 of the switch is a second linear function of the input voltagesignal 901 when the input voltage signal 901 is within a second range ofvalues 923. The duty cycle for a fixed value of feedback signal U_(FB)902 of the switch is an nth linear function of the input voltage signal901 when the input voltage signal is within an nth range of values 926.

In one example, slope 1 927 has a slope substantially equal to zerothough it is understood that in other examples slope 1 927 could alsohave a non-zero slope in accordance with the teachings of the presentinvention. In one example, all linear functions other than that whilethe input voltage signal 901 is within a first range of values 920 arelinear reduction of the duty cycle 902 for a given or fixed value offeedback signal value as the input voltage signal 901 increases. Inanother example, any one or more of the n slopes could have a slopesubstantially equal to zero as long as at least one of the n slopes hasa linear reduction of the duty cycle for a given or fixed value of thefeedback signal 902 as the input voltage signal 901 increases whilststill benefiting from the teachings of the present invention.

FIG. 10 shows generally an example schematic of a gain selector circuit1000 that in one example could be applied to generate the plurality oflinear functions of the input voltage signal as discussed above withreference to FIGS. 8 and 9 in accordance with the teachings of thepresent invention. The portion of gain selector circuit 1030 in oneexample shares many aspects with block 630 in FIG. 6 and is thereforenot discussed in detail here.

As shown, gain selector circuit 1000 is coupled to receive a feedforwardI_(FF) or input voltage signal 1010 at terminal 1051. Gain selectorcircuit 1000 generates a signal 1035 that is coupled to a switch dutycycle controller circuit, which in one example could be switch dutycycle controller 522 in FIG. 5. The gain selector circuit 1000 of FIG.10 includes a plurality of blocks to generate a plurality of linearfunctions of duty cycle adjust signal 1035 as a function of inputvoltage signal 1010.

For clarity of explanation purposes, only the nth block 1031 is shown inFIG. 10, which in one example could implement the nth slope 830 or 930in FIGS. 8 and 9 respectively. In the example, current I_(n) 1015 issubstantially zero until current I_(FF)/m1 1032 exceeds the currentI_(Bn) 1011 where I_(Bn)> . . . >I_(B2)>I_(B1). For values of I_(FF)/m11032 greater than I_(Bn) 1011, the difference between I_(FF)/m1 1032 andI_(Bn) 1011 is multiplied by current mirror 1020 ratio of mn andsubtracted from I₁ 1025. In general therefore the duty cycle adjustsignal 1035 can be represented by the following relationship:m _(FF) I _(FF) =I ₁ −I ₂ m2 . . . −I _(n) mn  (4)

The above description of illustrated examples of the present invention,including what is described in the Abstract, are not intended to beexhaustive or to be limitation to the precise forms disclosed. Whilespecific embodiments of, and examples for, the invention are describedherein for illustrative purposes, various equivalent modifications arepossible, as those skilled in the relevant art will recognize. Indeed,it is appreciated that the specific voltages, currents, frequencies,power range values, times, etc., are provided for explanation purposesand that other values may also be employed in other embodiments andexamples in accordance with the teachings of the present invention.

These modifications can be made to examples of the invention in light ofthe above detailed description. The terms used in the following claimsshould not be construed to limit the invention to the specificembodiments disclosed in the specification and the claims. Rather, thescope is to be determined entirely by the following claims, which are tobe construed in accordance with established doctrines of claiminterpretation.

1. A power supply controller, comprising: a switch duty cycle controllercoupled to receive a feedback signal and a duty cycle adjust signal, theswitch duty cycle controller coupled to generate a drive signal coupledto control switching of a switch coupled to an energy transfer elementto regulate energy delivered from an input of a power supply to anoutput of the power supply; and a gain selector circuit coupled toreceive an input voltage signal representative of an input voltage tothe power supply to generate the duty cycle adjust signal received bythe switch duty cycle controller, the switch duty cycle controller beingcoupled to limit a maximum duty cycle of the drive signal, wherein themaximum duty cycle of the drive signal is varied in response to aplurality of linear functions over a range of values of the inputvoltage signal.
 2. The power supply controller of claim 1 wherein theplurality of linear functions includes a first linear function, a secondlinear function and a third linear function, wherein the maximum dutycycle of the drive signal is the first linear function of the inputvoltage signal when the input voltage signal is below a first thresholdvalue, wherein the maximum duty cycle of the drive signal is a secondlinear function of the input voltage signal when the input voltagesignal is between the first threshold value and a second thresholdvalue, and wherein the maximum duty cycle of the drive signal is a thirdlinear function of the input voltage signal when the input voltagesignal is between the second threshold value and a third thresholdvalue.
 3. The power supply controller of claim 2 wherein a slope of thefirst linear function is substantially zero.
 4. The power supplycontroller of claim 2 wherein the second linear function is a linearreduction of the maximum duty cycle as the input voltage signalincreases.
 5. The power supply controller of claim 2 wherein the thirdlinear function is a linear reduction of the maximum duty cycle as theinput voltage signal increases.
 6. The power supply controller of claim2 wherein a slope of the third linear function is substantially zero. 7.The power supply controller of claim 1 wherein the input voltage signalis a current.
 8. The power supply controller of claim 1 wherein theinput voltage signal is a voltage.
 9. The power supply controller ofclaim 1 wherein the switch and the power supply controller are comprisedin a monolithic integrated circuit.
 10. The power supply controller ofclaim 1 wherein the switch and the power supply controller are comprisedin a hybrid integrated circuit.
 11. The power supply controller of claim1 wherein at least one of the plurality of linear functions over a rangeof values of the input voltage signal is a linear reduction of themaximum duty cycle as the input voltage signal increases.
 12. A powersupply controller, comprising: a switch duty cycle controller coupled toreceive a feedback signal and a duty cycle adjust signal, the switchduty cycle controller coupled to generate a drive signal coupled tocontrol switching of a switch coupled to an energy transfer element toregulate energy delivered from an input of a power supply to an outputof the power supply; and a gain selector circuit coupled to receive aninput voltage signal representative of an input voltage to the powersupply to generate the duty cycle adjust signal received by the switchduty cycle controller, the switch duty cycle controller being coupled tocontrol a duty cycle of the drive signal in response to both thefeedback signal and the duty cycle adjust signal, wherein the duty cycleof the drive signal for a fixed value of the feedback signal is variedin response to a plurality of linear functions over a range of values ofthe input voltage signal.
 13. The power supply controller of claim 12wherein the plurality of linear functions includes a first linearfunction, a second linear function and a third linear function, whereinthe duty cycle of the drive signal for the fixed value of the feedbacksignal is the first linear function of the input voltage signal when theinput voltage signal is below a first threshold value, wherein the dutycycle of the drive signal for the fixed value of the feedback signal isa second linear function of the input voltage signal when the inputvoltage signal is between the first threshold value and a secondthreshold value, and wherein the duty cycle of the drive signal for thefixed value of the feedback signal is a third linear function of theinput voltage signal when the input voltage signal is between the secondthreshold value and a third threshold value.
 14. The power supplycontroller of claim 13 wherein a slope of the first linear function issubstantially zero.
 15. The power supply controller of claim 13 whereinthe second linear function is a linear reduction of the duty cycle forthe fixed value of the feedback signal as the input voltage signalincreases.
 16. The power supply controller of claim 13 wherein the thirdlinear function is a linear reduction of the duty cycle for the fixedvalue of the feedback signal as the input voltage signal increases. 17.The power supply controller of claim 13 wherein a slope of the thirdlinear function is substantially zero.
 18. The power supply controllerof claim 12 wherein the input voltage signal is a current.
 19. The powersupply controller of claim 12 wherein the input voltage signal is avoltage.
 20. The power supply controller of claim 12 wherein the switchand the power supply controller are comprised in a monolithic integratedcircuit.
 21. The power supply controller of claim 12 wherein the switchand the power supply controller are comprised in a hybrid integratedcircuit.
 22. The power supply controller of claim 12 wherein at leastone of the plurality of linear functions over a range of values of theinput voltage signal is a linear reduction of the duty cycle for thefixed value of the feedback signal as the input voltage signalincreases.